CHANGE FEATURE: Variable description in dataset tree

Summary

The dataset variable descriptions shown in the dataset tree (implemented in 0cf03ba2) are useless. Instead it should only list the meta attributes and the open plots

Reason

If there is a long variable description, the text editor prevents scrolling the variable.

Current behaviour

The string representation of the variable is shown in a multi-line label. This causes problems when the user wants to scroll as it is not fluent

New behaviour

The xarray-representation should only be displayed in the tooltip. The tree should list the attributes, encoding and arrays that plot this variable